Audio/Video Transport Extensions A. Williams Internet-Draft Audinate Intended status: Standards Track February 2011 Expires: August 5, 2011 IEEE 1588/802.1AS Synchronisation for RTP Streams draft-williams-avtext-avbsync-00 Abstract Specification of an RTP header extension for carrying in-band synchronization metadata provided by the IEEE1588/802.1AS Precision Time Protocols. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on August 5, 2011. Copyright Notice Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect Williams Expires August 5, 2011 [Page 1] Internet-Draft AVB/RTP Synchronisation February 2011 to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Timestamp formats . . . . . . . . . . . . . . . . . . . . . . . 3 3. Header Extension . . . . . . . . . . . . . . . . . . . . . . . 4 4. IEEE 1733 / RTCP . . . . . . . . . . . . . . . . . . . . . . . 4 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5 7.1. Normative References . . . . . . . . . . . . . . . . . . . 5 7.2. Informative References . . . . . . . . . . . . . . . . . . 5 Appendix A. An Appendix . . . . . . . . . . . . . . . . . . . . . 5 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 5 Williams Expires August 5, 2011 [Page 2] Internet-Draft AVB/RTP Synchronisation February 2011 1. Introduction Synchronisation between RTP flows and between devices rendering RTP flows is currently facilitated by means of NTP format timestamps taken with respect to a shared reference clock. In professional AV applications, the NTP clock synchronisation protocol does not meet the time alignment and synchronisation speed requirements of many systems. Like NTP, the IEEE1588 family of clock synchronisation protocols provide a shared reference clock in an network - typically a LAN. IEEE1588 provides sub-microsecond synchronisation between devices on a LAN and typically locks within seconds at startup rather than minutes. With support from Ethernet switches, IEEE1588 protocols can achieve nanosecond timing accuracy in LANs. Network interface chips and cards supporting hardware time-stamping of timing critical protocol messages are also available. When using IEEE1588 clock synchronisation, networked AV systems can achieve sub 1 microsecond time alignment accuracy when rendering rendering of AV signals and can support latencies less than 1ms through a gigabit LAN. Two main flavours of IEEE1588 are in use today: o IEEE 1588-2008: the second version of the "Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems". This is a revised version of the original IEEE1588-2002 standard and is often called IEEE1588v2 or PTPv2. o IEEE 802.1AS: "Timing and Synchronization for Time Sensitive Applications in Bridged Local Area Networks". This is a Layer-2 only profile of IEEE 1588-2008 for use in Audio/Video Bridged LANs. By using an IEEE 1588 derived reference clock, synchronisation of RTP streams and devices in LANs can be considerably improved. 2. Timestamp formats A global IEEE 1588/802.1AS timestamp is 80 bits in total, divided into two parts: AS_sec: 48 bits seconds since epoch AS_nsec: 32 bits nanoseconds Williams Expires August 5, 2011 [Page 3] Internet-Draft AVB/RTP Synchronisation February 2011 A shorter 32 bit timestamp is defined for use in streaming media protocols in the following way: as_timestamp = (AS_sec * 10^9 + AS_nsec) modulo 2^32 The shorter as_timestamp field covers just over 4 seconds of time. 3. Header Extension 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |V=2|P|1| CC |M| PT | sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+R | timestamp |T +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+P | synchronisation source (SSRC) identifier | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | 0xBE | 0xDE | length=2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+E | ID-A | L=6 | subtype | reserved |x +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+t | as_timestamp |n +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | payload data | | .... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ IEEE 1588/802.1AS Synchronisation Header Extension The fields are defined as follows: subtype: IEEE 1733 RTCP subtype field, see Section 4. as_timestamp: a 32 bit IEEE 1588/802.1AS timestamp as defined in Section 4. reserved: as this specification evolves, additional fields are expected to be included in this header. 4. IEEE 1733 / RTCP IEEEE 1733 defines an RTCP header extension which also includes 802.1AS timestamp information. The IEEE 1733 RTCP extension signals infromation in addition to timestamps, including: Williams Expires August 5, 2011 [Page 4] Internet-Draft AVB/RTP Synchronisation February 2011 subtype an RTCP subtype gmIdentity an 80 bit field uniquely identifying the current 802.1AS grand master clock for the network stream_id a 64 bit number identifying the 802.1Qav stream associated with this RTP flow as_timestamp the 32 bit 802.1AS timestamp (Section 2) associated with the RTP timestamp carried in this packet rtp_timestamp the RTP timestamp of a media packet 5. IANA Considerations TBD: A URN will be required to signal the presence of this header extension, such as: urn:ietf:params:rtp-hdrext:avb-sync 6. Acknowledgements 7. References 7.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 7.2. Informative References [InfRef] "", 2004. Appendix A. An Appendix Williams Expires August 5, 2011 [Page 5] Internet-Draft AVB/RTP Synchronisation February 2011 Author's Address Aidan Williams Audinate Level 1, 458 Wattle St Ultimo, NSW 2007 Australia Phone: +61 2 8090 1000 Fax: +61 2 8090 1001 Email: aidan.williams@audinate.com Williams Expires August 5, 2011 [Page 6]